Method of performing cell search in wireless communication system

ABSTRACT

A method of performing cell search includes receiving a primary synchronization signal (PSS) comprising a primary synchronization code (PSC) and receiving a secondary synchronization signal (SSS) comprising a first secondary synchronization code (SSC) and a second SSC, wherein the SSS includes a first SSS and a second SSS, the first SSC and the second SSC are arranged in that order in the first SSS, and the second SSC and the first SSC are arranged in that order in the second SSS. Detection performance on synchronization signals can be improved, and cell search can be performed more reliably.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/654,582, filed Dec. 23, 2009 now U.S. Pat. No. 8,155,106, which is acontinuation of U.S. patent application Ser. No. 12/168,021, filed Jul.3, 2008 (now U.S. Pat. No. 7,808,967), and claims the benefit ofpriority of Korean Patent Application No. 10-2007-0068364 filed on Jul.6, 2007, Korean Patent Application No. 10-2007- 0072502 filed on Jul.19, 2007, Korean Patent Application No. 10-2007-0080129 filed on Aug. 9,2007, and Korean Patent Application No. 10-2007-0098861 filed on Oct. 1,2007, each of which is hereby incorporated by reference in its entiretyas if fully stated herein.

BACKGROUND

1. Technical Field

The present invention relates to wireless communication, and moreparticularly, to a method for performing cell search in a wirelesscommunication system.

2. Related Art

Wide code division multiple access (WCDMA) systems of the 3rd generationpartnership project (3GPP) use a total of 512 long pseudo-noise (PN)scrambling codes in order to identify base stations (BSs). As ascrambling code of a downlink channel, each BS uses a different long PNscrambling code.

When power is supplied to a user equipment (UE), the UE performsdownlink synchronization of a cell and acquires a long PN scramblingcode identifier (ID) of the cell. Such a process is generally referredto as a cell search. The cell search is the procedure by which a userequipment acquires time and frequency synchronization with a cell anddetects the cell identity of the cell. The initial cell is determinedaccording to a location of the UE at a time when the power is supplied.In general, the initial cell indicates a cell of a BS corresponding tothe greatest one of signal components of all BSs, which are included ina downlink reception signal of the UE.

To facilitate the cell search, a WCDMA system divides 512 long PNscrambling codes into 64 code groups, and uses a downlink channelincluding a primary synchronization channel (P-SCH) and a secondarysynchronization channel (S-SCH). The P-SCH is used to allow the UE toacquire slot synchronization. The S-SCH is used to allow the UE toacquire frame synchronization and a scrambling code group.

In general, cell search is classified into initial cell search, which isinitially performed when a UE is powered on, and non-initial searchwhich performs handover or neighbor cell measurement.

In the WCDMA system, the cell search is accomplished in three steps. Inthe first step, a UE acquires slot synchronization by using a P-SCHincluding a primary synchronization code (PSC). A frame includes 15slots, and each BS transmits the frame by including a PSC. Herein, thesame PSC is used for the 15 slots, and all BSs use the same PSC. The UEacquires the slot synchronization by using a matched filter suitable forthe PSC. In the second step, a long PN scrambling code group and framesynchronization are acquired by using the slot synchronization and alsoby using a S-SCH including a secondary synchronization code (SSC). Inthe third step, by using a common pilot channel code correlator on thebasis of the frame synchronization and the long PN scrambling codegroup, the UE detects a long PN scrambling code ID corresponding to along PN scrambling code used by the initial cell. That is, since 8 longPN scrambling codes are mapped to one long PN scrambling code group, theUE computes correlation values of all of the 8 long PN scrambling codesbelonging to a code group of the UE. On the basis of the computationresult, the UE detects the long PN scrambling code ID of the initialcell.

Since the WCDMA system is an asynchronous system, only one PSC is usedin the P-SCH. However, considering that a next generation wirelesscommunication system has to support both synchronous and asynchronousmodes, there is a need for using a plurality of PSCs.

If errors occur while detecting the S-SCH, delay occurs when a UEperforms cell search. Therefore, there is a need to improve channeldetection performance in the cell search procedure.

SUMMARY

A method is sought for improving detection performance by performingscrambling in such a manner that different scrambling codes are used fora secondary synchronization signal.

A method is also sought for performing a reliable cell search byimproving detection performance on the secondary synchronization signal.

A method is also sought for transmitting synchronization signals byimproving detection performance on the synchronization signals.

In an aspect, a method of performing cell search in a wirelesscommunication system is provided. The method includes receiving aprimary synchronization signal (PSS) comprising a primarysynchronization code (PSC), acquiring an unique identity from the PSS,receiving a secondary synchronization signal (SSS) which is associatedwith a cell identity group, the SSS comprising a first secondarysynchronization code (SSC) and a second SSC, and acquiring a cellidentity which is defined by the unique identity within the cellidentity group, wherein the SSS includes a first SSS and a second SSS,the first SSC and the second SSC are arranged in that order in the firstSSS, and the second SSC and the first SSC are arranged in that order inthe second SSS, wherein the first SSC and the second SSC arerespectively scrambled by using two different scrambling codes.

The first SSC of the first SSS can be scrambled by using a firstscrambling code, the second SSC of the first SSS can be scrambled byusing a second scrambling code, the first SSC of the second SSS can bescrambled by using the second scrambling code, and the second SSC of thesecond SSS can be scrambled by using the first scrambling code.

The first SSC and the second SSC can be defined by two different cyclicshifts of a m-sequence generated by a generating polynomial x⁵+x²+1. Thefirst scrambling code and the second scrambling code can be defined bytwo different cyclic shifts of a m-sequence generated by a generatingpolynomial x⁵+x³+1.

In another aspect, a method of transmitting synchronization signals in awireless communication system is provided. The method includestransmitting a PSS comprising a PSC, transmitting a first SSS comprisinga first SSC and a second SSC, and transmitting a second SSS comprisingthe first SSC and the second SSC, wherein the first SSC and the secondSSC are respectively scrambled by using a first scrambling code and asecond scrambling code, wherein the locations of the first and secondSSCs of the first SSS are swapped with those of the first and secondSSCs of the second SSS.

In still another aspect, a method of acquiring synchronization signalsin a wireless communication system is provided. The method includesidentifying a PSC by a PSS transmitted from a base station, andidentifying a first SSC and a second SSC by a SSS transmitted from thebase station, wherein the SSS includes a first SSS and a second SSS, thefirst SSC and the second SSC are arranged in that order in the firstSSS, and the second SSC and the first SSC are arranged in that order inthe second SSS, wherein the first SSC and the second SSC arerespectively scrambled by using two different scrambling codes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a wireless communication system.

FIG. 2 shows an example of a radio frame structure.

FIG. 3 shows an example of physical mapping of two SSCs onto a SSS.

FIG. 4 shows another example of physical mapping of two SSCs onto a SSS.

FIG. 5 shows an example of mapping of two SSCs onto a SSS.

FIG. 6 shows another example of mapping of two SSCs onto a SSS.

FIG. 7 shows a SSS structure according to an embodiment of the presentinvention.

FIG. 8 shows a SSS structure according to another embodiment of thepresent invention.

FIG. 9 shows a SSS structure according to another embodiment of thepresent invention.

FIG. 10 shows a SSS structure according to another embodiment of thepresent invention.

FIG. 11 shows a SSS structure according to another embodiment of thepresent invention.

FIG. 12 shows a SSS structure for a PSC 1.

FIG. 13 shows a SSS structure for a PSC 2.

FIG. 14 shows a SSS structure for a PSC 3.

FIG. 15 shows a SSS structure for a PSC 1.

FIG. 16 shows a SSS structure for a PSC 2.

FIG. 17 shows a SSS structure for a PSC 3.

FIG. 18 is a graph showing a cumulative distribution function (CDF) ofcross-correlation distribution for all possible collisions in two cells.

FIG. 19 is a flowchart showing cell search according to an embodiment ofthe present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a structure of a wireless communication system. Thewireless communication system can be widely deployed to provide avariety of communication services, such as voices, packet data, etc.

Referring to FIG. 1, a wireless communication system includes a userequipment (UE) 10 and a base station (BS) 20. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc. The BS 20 is generally a fixed station thatcommunicates with the UE 10 and may be referred to as anotherterminology, such as a node-B, a base transceiver system (BTS), anaccess point, etc. There are one or more cells within the coverage ofthe BS 20.

The wireless communication system may be an orthogonal frequencydivision multiplexing (OFDM)/orthogonal frequency division multipleaccess (OFDMA)-based system. The OFDM uses a plurality of orthogonalsubcarriers. Further, the OFDM uses an orthogonality between inversefast Fourier transform (IFFT) and fast Fourier transform (FFT). Atransmitter transmits data by performing IFFT. A receiver restoresoriginal data by performing FFT on a received signal. The transmitteruses IFFT to combine the plurality of subcarriers, and the receiver usesFFT to split the plurality of subcarriers.

I. Sequence Generation

According to an embodiment of the present invention, a pseudo-noise

(PN) sequence is used as a sequence applied to a secondarysynchronization signal (SSS). The PN sequence can be reproduced andshows a characteristic similar to a random sequence. The PN sequence ischaracterized as follows. (1) A repetition period is sufficiently long.If a sequence has an infinitely long repetition period, the sequence isa random sequence. (2) The number of 0s is close to the number of 1swithin one period. (3) A portion having a run length of 1 is ½, aportion having a run length of 2 is ¼, a portion having a run length of3 is ⅛, and so on. Herein, the run length is defined as the number ofcontiguous identical symbols. (4) A cross-correlation between sequenceswithin one period is significantly small. (5) A whole sequence cannot bereproduced by using small sequence pieces. (6) Reproducing is possibleby using a proper reproducing algorithm.

A PN sequence includes an m-sequence, a gold sequence, a Kasamisequence, etc. For clarity, the m-sequence will be described as anexample. In addition to the aforementioned characteristic, them-sequence has an additional characteristic in which a side lobe of aperiodic auto-correlation is −1.

An example of a generating polynomial for generating an m-sequence c_(k)can be expressed asc _(k) =x ⁵ +x ²+1 over GF(2)   [Equation 1]

where GF denotes a Galois Field, and GF(2) represents a binary signal.

A maximum length generated by Equation 1 is 2⁵−1=31. In this case,according to a generated state, a total of 3 sequences can be generated.This coincides with a maximum number of sequences (i.e., 31) that can begenerated by using a cyclic shift after an arbitrary m-sequence isgenerated by Equation 1. This means that a maximum of 31 pieces ofinformation can be transmitted. Even if the information is simple, morethan 31 pieces of information cannot be transmitted.

According to another embodiment of the present invention, if anm-sequence is defined as d(n), a sequence set S1 for all availablesequences can be expressed as S1={d^(m)(k)|m is a sequence index} wherem=0, 1, . . . , N-1 and k=0, 1, . . . , N-1. N is N=2^(n)−1 where n is amaximum degree. For example, in the case of the generating polynomial ofEquation 1, n=5 and N=31.

A new sequence g^(m)(k) is defined by g^(m)(k)=d^(m)(N-1-k), m=0, 1, . .. , N-1, k=0, 1, . . . , N-1. A sequence set S2 is defined byS2={g^(m)(k)|m is a sequence index}. A sequence set S3 can be defined byS3={S1,S2}. Characteristics of the m-sequence are kept in S1 and S2. Arandom sequence property is maintained between S1 and S2. Therefore, asequence having a good correlation property can be generated within acorresponding sequence set, and the number of available sequences can beincreased without using an additional memory or without increasingoverhead.

In particular, the m-sequence can be generated by an n-th degreepolynomial as showna ₀ x ^(n) +a ₁ x ^(n-1) + . . . +a _(n-1)1   [Equation 2]

where k=0, 1, . . . , n-1, and a_(k)=0 or 1.

By using the definition of the sequence g^(m)(k), the m-sequence can beconverted into one of m-sequences generated as showna _(n-1) x ^(n-0) +a _(n-2) x ^(n-1) + . . . +a ₀ x ^(n-m) =a _(n-1) x^(n) +a _(n-2) x ^(n-1) + . . . +a ₀1   [Equation 3]

where k=0, 1, . . . , n-1, and a_(k)=0 or 1. This means thatcoefficients of the generating polynomial are reversed in comparisonwith Equation 2. This also means that the sequences generated byEquation 2 are reversed in order. In this case, it is said that the twoEquations are in a reverse relationship. The reverse relationship isalso satisfied when a degree of a polynomial is reversed (herein, thedegree of the polynomial is modified to n-k). When using them-sequences, the polynomial can be selected to satisfy the reverserelationship.

For example, if n=5, the polynomial for generating the m-sequences canbe expressed as shown[Equation 4]x⁵+x²+1   (1)x⁵+x³+1   (2)x⁵+x³+x²+x¹+1   (3)x⁵+x⁴+x³′x²+1   (4)x⁵+x⁴+x²+x¹+1   (5)x⁵+x⁴+x³+x¹+1   (6)

In this case, (1) and (2), (3) and (4), and (5) and (6) are in a pairrelationship which satisfies the reverse relationship expressed byEquations 2 and 3. The m-sequences can be selected to satisfy thereverse relationship.

When a significantly long sequence is used, the sequence may be dividedinto several pieces by differently determining a start offset of thesequence. In this case, each piece of sequence can be used in a reverseorder.

In addition, when the significantly long sequence is used, the longsequence may be reversed, and then the reversed sequence can be dividedinto several pieces by differently determining a start offset of thesequence.

The aforementioned sequence can be used in several channels. The greaterthe number of available sequences, the higher the capacity of UEs.

In an embodiment, the aforementioned sequence is used in asynchronization signal. Further, the sequence is used in a primarysynchronization code (PSC) for a primary synchronization signal (PSS) orin a secondary synchronization code (SSC) for a secondarysynchronization signal (SSS). Furthermore, the sequence is used in ascrambling code. In this case, the sequence can be selected so that theSSC and the scramble code satisfy a reverse relationship.

In anther embodiment, the aforementioned sequence is used in a randomaccess preamble. The random access preamble is used for request ofuplink radio resources. One sequence index corresponds to oneopportunity. A UE randomly selects any one of sequence sets and thusinforms a BS of the existence of the UE, or performs an operation suchas scheduling request or bandwidth request. A random access procedure isa contention-based procedure. Thus, collision may occur among UEs. Toreduce the collision among the UEs in the random access procedure, thenumber of random access preambles in the set needs to be large enough.For example, if the random access preambles are configured by usingEquation 1, there are 31 opportunities. If the random access preamblesare configured by using the definition of the sequence S3, there are 62opportunities.

In still another embodiment, the aforementioned sequence can be used totransmit a channel quality indicator (CQI) or an acknowledgment(ACK)/negative-acknowledgement (NACK) signal. When the sequence ofEquation 1 is used, a total of 31 CQI or ACK/NACK signal (>4 bits) canbe transmitted. When the sequence S3 is used, a total of 62 CQI orACK/NACK signal (>5 bits) can be transmitted.

In still another embodiment, the aforementioned sequence can be used toa base sequence for a reference signal. The reference signal may beclassified into a demodulation reference signal for data demodulation ora sounding reference signal for uplink scheduling. The reference signalneeds to have a large number of available sequences to facilitate cellplanning and coordination. For example, assume that a total of 170sequences are required as a downlink reference signal. Then, when abandwidth of 1.25 MHz is used as a reference, the number of subcarriersoccupied by the reference signal is 120 within an OFDM symbol length of5 ms. If an m-sequence is used, a total of 127 sequences can begenerated by using a 7-th degree polynomial. When using the sequence S3,a total of 252 sequences can be generated. Assume that the uplinkreference signal is assigned to one resource block including 12subcarriers. Then, when the m-sequence is used, a total of 15 sequencescan be generated by using a 4-th degree polynomial. When using thesequence S3, a total of 30 sequences can be generated.

II. Synchronization Signal

Now, a synchronization signal will be described. Technical features ofthe present invention can be easily applied to a random access preambleor other control signals by those ordinary skilled in the art.

FIG. 2 shows an example of a radio frame structure.

Referring to FIG. 2, a radio frame includes 10 sub-frames. One sub-frameincludes two slots. One slot includes a plurality of OFDM symbols intime domain. Although one slot includes 7 OFDM symbols in FIG. 2, thenumber of OFDM symbols included in one slot may vary depending on acyclic prefix (CP) structure.

The radio frame structure is for exemplary purposes only. Thus, thenumber of sub-frames and the number of slots included in each sub-framemay vary in various ways.

A primary synchronization signal (PSS) is transmitted in last OFDMsymbol in each of a 0-th slot and a 10-th slot. The same PSC is used bytwo PSSs. The PSS is used to acquire OFDM symbol synchronization (orslot synchronization) and is associated with a unique identity in a cellidentity group. The PSC may be generated from a Zadoff-Chu (ZC)sequence. At least one PSC exists in a wireless communication system.

The PSS comprise a primary synchronization code (PSC). When three PSCsare reserved, a BS selects one of the three PSCs, and transmits theselected PSC in the last OFDM symbols of the 0-th slot and the 10-thslot as the PSS.

A secondary synchronization signal (SSS) is transmitted in OFDM symbolwhich is immediately previously located from the OFDM symbol for thePSS. This means that the SSS and the PSS are transmitted in contiguous(or consecutive) OFDM symbols. The SSS is used to acquire framesynchronization and is associated with a cell identity group. The cellidentity can uniquely defined by the cell identity group acquired fromthe SSS and the unique identity acquired from the PSS. The UE canacquire the cell identity by using the PSS and the SSS.

One SSS comprises two secondary synchronization codes (SSCs). One SSCmay use a PN sequence (i.e., m-sequence). For example, if one SSSincludes 64 subcarriers, two PN sequences having a length of 31 aremapped to the one SSS.

A location or the number of OFDM symbols in which the PSS and the SSSare arranged over a slot is shown in FIG. 2 for exemplary purposes only,and thus may vary depending on a system.

FIG. 3 shows an example of physical mapping of two SSCs onto a SSS.

Referring to FIG. 3, if the number of subcarriers included in the SSS isN, a length of a first SSC SSC1 and a length of a second SSC SSC2 areN/2. A logical expression indicates an SSC in use. A physical expressionindicates subcarriers to which an SSC is mapped when the SSC istransmitted in the SSS. S1(n) denotes an n-th element of the first SSCSSC1. S2(n) denotes an n-th element of the second SSC SSC2. The firstSSC SSC1 and the second SSC SSC2 are interleaved to each other, and aremapped to physical subcarriers in a comb-type configuration. Such amapping method is referred to as distributed mapping.

FIG. 4 shows another example of physical mapping of two SSCs onto a SSS.

Referring to FIG. 4, the number of subcarriers included in the SSS is N.A length of a first SSC SSC1 and a length of a second SSC SSC2 are N/2.A logical expression indicates an SSC in use. A physical expressionindicates subcarriers to which an SSC is mapped when the SSC istransmitted in the SSS. S1(n) denotes an n-th entity of the first SSCSSC1. S2(n) denotes an n-th entity of the second SSC SSC2. The first SSCSSC1 and the second SSC SSC2 are mapped to locally concentrated physicalsubcarriers. Such a mapping method is referred to as localized mapping.

If the number of subcarriers in the SSS is 62, and the length of the PNcode is 31, then one SSC has a total of 31 indices. If the first SSCSSC1 can have indices 0 to 30, and the second SSC SSC2 can have indices0 to 30, then a total of 961 (i.e., 31×31=961) pieces of information canbe delivered.

III. Mapping of SSC onto SSS

FIG. 5 shows an example of mapping of two SSCs onto a SSS.

Referring to FIG. 5, since two SSSs are transmitted in a radio frame asshown in FIG. 2, a first SSS assigned to a 0-th slot and a second SSSassigned to a 10-th slot both use a combination of a first SSC SSC1 anda second SSC SSC2. In this case, locations of the first SSC SSC1 and thesecond SSC SSC2 are swapped with each other in frequency domain. Thatis, when a combination of (SSC1, SSC2) is used in the first SSS, thesecond SSS swaps the first SSC SSC1 and the second SSC SSC2 with eachother and thus uses a combination of (SSC2, SSC1).

To detect the SSSs, an interval between the first SSS and the second SSScan be predetermined. Multi-frame averaging can be performed accordingto the CP structure. The multi-frame averaging is an operation in whicha plurality of SSSs are received by using a plurality of radio framesand then values acquired from the respective SSSs are averaged. If theCP structure is not known, the multi-frame averaging is performed forall CP structures. A structure of swapping SSCs is advantageous when areceiver detects the SSSs by performing the multi-frame averaging. Inthis structure, the first SSS and the second SSS use the samecombination of SSCs, and there is no change except for the locations ofthe SSCs. Thus, when the averaging is performed, the second SSS simplyswaps and integrates the SSCs. On the other hand, when a structure ofnot swapping SSCs is used, even if coherent detection is performed usinga PSS, non-coherent combining has to be performed when the detectionresults are averaged. However, when the coherent detection using the PSSis performed, performance improvement can be expected since optimalmaximal ratio combining (MRC), i.e., coherent combining, can beperformed when the SSCs are integrated. It is well-known that the MRC isthe optimal combining. In general, there is a gain of about SNR of 3 dBin the coherent combining over the non-coherent combining.

Although the first SSC SSC1 and the second SSC SSC2 are swapped in thefirst SSS and the second SSS in the frequency domain, this is forexemplary purposes only. Thus, the first SSC SSC1 and the second SSCSSC2 may be swapped in time domain or code domain.

FIG. 6 shows another example of mapping of two SSCs onto a SSS. Herein,binary phase shift keying (BPSK) modulation is used. The BPSK modulationis M-phase shift keying (PSK) modulations when M=2. In the BPSKmodulation, the whole or some parts of a channel are modulated into +1or −1. By using the M-PSK modulation, additional information can becarried without having an effect on detection performance of a sequencecurrently being used.

Referring to FIG. 6, a first SSS and a second SSS both use a combinationof a first SSC SSC1 and a second SSC SSC2, modulate the whole parts ofthe first SSS into +1, modulate a first SSC SSC1 of the second SSS into+1, and modulate a second SSC SSC2 of the second SSS into −1. That is,modulation can be performed by changing phases between SSCs used in oneSCH, or can be performed by changing phases between two SCHs. This iscalled differential modulation.

In general, to detect sequences which have undergone modulation, asignal (i.e., a reference signal or a PSC) is required for a phasereference. That is, coherence detection is required. However, when thedifferential modulation is performed to identify a frame boundary withinone SSS, both the coherent detection and the non-coherent detection arepossible.

IV. Scrambling of SSS

Now, scrambling of a SSS by using a scrambling code associated with aPSC will be described.

The SSS is scrambled by using the scrambling code. The scrambling codeis a binary sequence associated with the PSC and is one-to-one mapped tothe PSC. In other words, the scrambling code depends on the PSC.

The scrambling of the SSS is used to solve ambiguity resulted from SSCdetection. For example, assume that an SSC combination used in a SSS ofa cell A is (SSC1, SSC2)=(a,b), and an SSC combination used in a SSS ofa cell B is (SSC1, SSC2)=(c,d). In this case, if a UE belonging to thecell A acquires a wrong SSC combination, that is, (SSC1, SSC2)=(a,d),this is called ambiguity. That is, after the UE detects a PSS, thescrambling code is used to facilitate distinction of the SSScorresponding to the cell of the UE.

FIG. 7 shows a SSS structure according to an embodiment of the presentinvention.

Referring to FIG. 7, a first SSS and a second SSS both use a combinationof a first SSC SSC1 and a second SSC SSC2. In this case, locations ofthe first SSC SSC1 and the second SSC SSC2 are swapped in frequencydomain. That is, when a combination of (SSC1, SSC2) is used in the firstSSS, the second SSS swaps the first SSC SSC1 and the second SSC SSC2with each other and thus uses a combination of (SSC2, SSC1).

The SSCs of the respective SSSs are scrambled by using differentscrambling codes. The first SSC SSC1 of the first SSS is scrambled by afirst scrambling code. The second SSC SSC2 of the first SSS is scrambledby a second scrambling code. The second SSC SSC2 of the second SSS isscramebled by a third scrambling code. The first SSC SSC1 of the secondSSS is scrambled by a fourth scrambling code.

Since each SSC is scrambled by a different scrambling code, aninterference averaging effect can be achieved. For example, assume thatan SSC combination of the first SSS of a cell A is (SSC1_A, SSC2_A)=(a,b), a SSC combination of the second SSS of the cell A is (SSC2_A,SSC1_A)=(b, a), a SSC combination of the first SSS of a cell B is(SSC1_B, SSC2_B)=(c, d), a SSC combination of the second SSS is (SSC2_B,SSC1_B)=(d, c), the cell A is a cell where a UE is currently located(that is, the cell A is a cell to be detected), and the cell B is aneighbor cell (that is, the cell B is a cell which acts asinterference). Then, interference of SSC1_A and interference of SSC2_Aare c and d and thus become equal irrespective of the first SSS and thesecond SSS. Therefore, the interference averaging effect cannot beachieved. However, when each SSC is scrambled by using a differentscrambling code, the interference averaging effect can be achieved dueto an interference effect of different codes.

Therefore, since different scrambling codes are used for the same SSCfor each sub-frame, ambiguity resulting from SSC detection can bereduced. Further, the interference averaging effect can be achieved whenmulti-frame averaging is performed.

Herein, the SSC structure represents a logical structure. When mappingis performed on physical subcarriers, distributed mapping or localizedmapping may be used. In addition, physical mapping may be performedbefore or after scrambling is performed in the logical structure.

FIG. 8 shows a SSS structure according to another embodiment of thepresent invention.

Referring to FIG. 8, a first SSS and a second SSS both use a combinationof a first SSC SSC1 and a second SSC SSC2. In this case, locations ofthe first SSC SSC1 and the second SSC SSC2 are swapped in frequencydomain. That is, when a combination of (SSC1, SSC2) is used in the firstSSS, the second SSS swaps the first SSC SSC1 and the second SSC SSC2with each other and thus uses a combination of (SSC2, SSC1).

Scrambling is performed by using two scrambling codes, corresponding tothe number of SSCs included in one SSS. The first SSC SSC1 of the firstSSS is scrambled by a first scrambling code. The second SSC SSC2 of thefirst SSS is scrambled by a second scrambling code. The second SSC SSC2of the second SSS is scrambled by the first scrambling code. The firstSSC SSC1 of the second SSS is scrambled by the second scrambling code.

From the viewpoint of a physical expression in which mapping is made toactual subcarriers, two SSCs swap their locations for the first SSS andthe second SSS but the locations of the scrambling codes are notswapped. From the viewpoint of a logical expression, the scramblingcodes respectively applied to the first SSC SSC1 and the second SSC SSC2have an effect so that the scrambling codes respectively applied to thesecond SSC SSC2 and the first SSC SSC1 of the second SSS are changed. Incomparison with the embodiment of FIG. 7, the number of requiredscrambling codes decreases.

FIG. 9 shows a SSS structure according to another embodiment of thepresent invention.

Referring to FIG. 9, a first SSS and a second SSS use the samecombination of a first SSC SSC1 and a second SSC SSC2. That is, if thefirst SSS uses a combination of (SSC1, SSC2), the second SSS also usesthe combination of (SSC1, SSC2). Locations of the first SSC SSC1 and thesecond SSC SSC2 are not swapped with each other in frequency domain. Inthe frequency domain, the locations of the first SSC SSC1 and the secondSSC SSC2 are equal to each other in the first SSS and the second SSS.

Scrambling is performed by using two scrambling codes, corresponding tothe number of SSCs included in one SSS. In this case, the locations ofthe scrambling codes used for the first SSS and the second SSS areswapped with each other. The first SSC SSC1 of the first SSS uses afirst scrambling code. The second SSC SSC2 of the first SSS uses asecond scrambling code. The second SSC SSC2 of the second SSS uses thesecond scrambling code. The first SSC SSC1 of the second SSS uses thefirst scrambling code.

Unlike the embodiment of FIG. 8, the SSCs do not swap their locationsfor the first SSS and the second SSS, but do swap the locations of thescrambling codes. That is, for the first SSS and the second SSS, thelocations of the SSCs or the scrambling codes are swapped with eachother.

FIG. 10 shows an SSS structure according to another embodiment of thepresent invention.

Referring to FIG. 10, in frequency domain, a first SSC SSC1 and a secondSSC SSC2 have the same location in a first SSS and a second SSS, exceptthat the second SSC of the second SSS is −SSC2. That is, the first SSSuses (SSC1, SSC2), and the second SSS uses (SSC1, −SSC2).

A modulation scheme used herein is BPSK modulation. A higher-ordermodulation scheme may also be used. For example, when Quadrature phaseshift key (QPSK) modulation is used, it is possible to change a phase byperforming modulation in a form of +1, −1, +j, −j. The first SSS may use(SSC1, SSC2), and the second SSS uses (SSC1, −jSSC2).

It is difficult to perform interference randomization if an SSCcombination of the first SSS is equal to an SSC combination of thesecond SSS in a multi-cell environment. Thus, if the first SSC SSC1 andthe second SSC −SSC2 are not swapped with each other, scrambling codesare swapped with each other. In this case, differential modulationinformation of the first SSC SSC1 and the second SSC −SSC2 in the secondSSS can represent frame boundary information. Therefore, in order todetect 392(=14×14×2) signals, a detection operation is performed 392times when differential modulation is not performed. On the other hand,when the differential modulation is performed, the detection operationis performed 196(=14*14) times, and two pieces of information can bedetected by using the differential modulation. An overall detectionperformance is determined by the detection operation performed 196 timesrather than the differential modulation. Thus, the overall performancecan be further improved when the differential modulation is performed.In addition, since the first SSS and the second SSS both use the samefirst and second scrambling codes SSC1 and SSC2, MRC combination can beperformed.

Although differential modulation is used for the second SSC −SSC2 of thesecond SSS, this is for exemplary purposes only. For example, the firstSSS may use (SSC1, SSC2), and the second SSS may use (−SSC1, −SSC2). Thefirst SSS may use (SSC1, SSC2), and the second SSS may use (−SSC1,SSC2). The first SSS may use (−SSC1, SSC2), and the second SSS may use(SSC1, −SSC2). The first SSS may use (SSC1, −SSC2), and the second SSSmay use (−SSC1, SSC2). The first SSS may use (−SSC1, −SSC2), and thesecond SSS may use (SSC1, SSC2). The first SSS may use (SSC1, −SSC2),and the second SSS may use (SSC1, SSC2). The first SSS may use (−SSC1,SSC2), and the second SSS may use (SSC1, SSC2). In addition thereto,various other modulation combinations may also be used.

FIG. 11 shows an SSS structure according to another embodiment of thepresent invention.

Referring to FIG. 11, a first SSS and a second SSS both use acombination of a first SSC SSC1 and a second SSC SSC2. In this case,locations of the first SSC SSC1 and the second SSC SSC2 are swapped in afrequency domain. That is, when a combination of (SSC1, SSC2) is used inthe first SSS, the second SSS swaps the first SSC SSC1 and the secondSSC SSC2 with each other and thus uses a combination of (SSC2, SSC1).The first SSC of the second SSS is differential-modulated into −SSC1.That is, the first SSS uses (SSC1, SSC2), and the second SSS uses (SSC2,−SSC1).

V. Scrambling when a Plurality of PSCs are Used

Now, an example of configuring a scrambling code when a plurality ofPSCs are used will be described. For clarity, it is assumed that threePSCs are used, and scrambling codes associated with the respective PSCsare defined as Px-a1, Px-a2, Px-b1, and Px-b2, respectively. Herein, ‘x’denotes a PSC index, ‘a’ denotes a first SSS, ‘b’ denotes a second SSS,‘1’ denotes a first SSC SSC1, and ‘2’ denotes a second SSC SSC2. Thatis, P1-a1 denotes a scrambling code associated with a first PSC and usedin the first SSC SSC1 of the first SSS, P2-b2 denotes a scrambling codeassociated with a second PSC and used in the second SSC SSC2 of thesecond SSS, P3-a1 denotes a scrambling code associated with a third PSCand used in the first SSC SSC1 of the first SSS. When it is said that ascrambling code is associated with a PSC, it means that the scramblingcode is generated differently according to the PSC. For example, thescrambling code may be generated by using a different cyclic shiftaccording to a cell identifier (ID) using the PSC.

<In Case of Using 6 Scrambling Codes for 3 PSCs>

For each PSC, scrambling codes may be configured such as (Px-a1,Px-a2)=(Px-b1, Px-b2). (Px-a1, Px-a2) is one-to-one mapped to therespective PSCs. That is, for the three PSCs, six scrambling codes canbe defined as follows.

PSC 1→(P1-a1, P1-a2)

PSC 2→(P2-a1, P2-a2)

PSC 3→(P3-a1, P3-a2)

FIG. 12 shows a SSS structure for a PSC 1. FIG. 13 shows a SSS structurefor a PSC 2. FIG. 14 shows a SSS structure for a PSC 3.

Referring to FIGS. 12 to 14, for each PSC, a first SSS and a second SSSboth use a combination of a first SSC SSC1 and a second SSC SSC2. Inthis case, locations of the first SSC SSC1 and the second SSC SSC2 areswapped. That is, if the first SSS uses a combination of (SSC1, SSC2),the second SSS swaps the first SSC SSC1 and the second SSC SSC2 witheach other and thus uses a combination of (SSC2, SSC1).

Scrambling is performed by using two scrambling codes, corresponding tothe number of SSCs included in one SSS.

In the PSC 1 of FIG. 12, the first SSC SSC1 of the first SSS uses ascrambling code P1-a1, the second SSC SSC2 of the first SSS uses ascrambling code P1-a2, the second SSC SSC2 of the second SSS uses ascrambling code P1-a1, and the first SSC SSC1 of the second SSS uses ascrambling code P1-a2.

In the PSC 2 of FIG. 13, the first SSC SSC1 of the first SSS uses ascrambling code P2-a1, the second SSC SSC2 of the first SSS uses ascrambling code P2-a2, the second SSC SSC2 of the second SSS uses ascrambling code P2-a1, and the first SSC SSC1 of the second SSS uses ascrambling code P2-a2.

In the PSC 3 of FIG. 14, the first SSC SSC1 of the first SSS uses ascrambling code P3-a1, the second SSC SSC2 of the first SSS uses ascrambling code P3-a2, the second SSC SSC2 of the second SSS uses ascrambling code P3-a1, and the first SSC SSC1 of the second SSS uses ascrambling code P3-a2.

When mapping is performed on a physical channel, the two SSCs swap theirlocations for the first SSS and the second SSS, but do not swaplocations of the scrambling codes.

In this method, scrambling codes associated with three PSCs aredifferent from one another with respect to both the first SSC SSC1 andthe second SSC SSC2. This can reduce ambiguity and also bring aninterference randomization effect. For example, assume that an SSCcombination of a first SSS of a cell A is (P1-a1

SSC1_A, P1-a2

SSC2_A), an SSC combination of a second SSS of the cell A is (P1-a1

SSC2_A, P1-a2

SSC1_A), an SSC combination of a first SSS of a cell B is (P2-a1

SSC1_B, P2-a2

SSC2_B), an SSC combination of a second SSS of the cell B is (P2-a1

SSC2_B, P2-a2

SSC1_B), the cell A is a cell where a UE is currently located, and thecell B is a neighbor cell. Then, interference of the first SSS of thecell A is (P2-a1

SSC1_B, P2-a2

SSC2_B), and interference of the second SSS is (P2-a1

SSC2_B, P2-a2

SSC1_B). In practice, since a different code acts as interference to thefirst SSC SSC1 and the second SSC SSC2 with respect to the first SSS andthe second SSS, advantages of an interference averaging effect andmulti-frame averaging can be achieved without deterioration.Accordingly, detection performance on the SSSs can be improved.

<In Case of Using 3 Scrambling Codes for 3 PSCs>

For each PSC, scrambling codes may be configured such as (Px-a1,Px-a2)=(Px-b1, Px-b2). (Px-a1, Px-a2) is one-to-one mapped to therespective PSCs. One of the two scrambling codes mapped to one PSS isequal to one of scrambling codes mapped to another PSS. For example, arelationship of Px_a2=P[mod(x+1,3)+1]_a1 is maintained. Herein, ‘mod’denotes modulo operation. For example, three scrambling codes for threePSCs can be defined as follows.

PSC 1→(P1-a1, P1-a2)

PSC 2→(P2-a1=P1-a2, P2-a2)

PSC 3→(P3-a1=P2-a2, P3-a2=P1-a1)

Three scrambling codes P1-a1, P1-a2, and P2-a2 are required in practice.If (P1-a1, P1-a2, P2-a2)=(a₁,a₂,a₃), the three scrambling codes can beexpressed as follows.

PSC 1→(a₁, a₂)

PSC 2→(a₂, a₃)

PSC 3→(a₃, a₁)

The number of required scrambling codes can be reduced bycyclic-shifting the three scrambling codes for the respective PSCs. Byreducing the number of scrambling codes, memory capacity of a BS or a UEcan be saved.

If M PSCs are used, the scrambling codes can be generalized as follows.

PSC 1→(a₁, a₂)

PSC 2→(a₂, a₃)

PSC M→(a_(M), a₁) FIG. 15 shows a SSS structure for a PSC 1. FIG. 16shows a SSS structure for a PSC 2. FIG. 17 shows a SSS structure for aPSC 3.

Referring to FIGS. 15 to 17, for each PSC, a first SSS and a second SSSboth use a combination of a first SSC SSC1 and a second SSC SSC2. Inthis case, locations of the first SSC SSC1 and the second SSC SSC2 areswapped in a frequency domain. That is, when a combination of (SSC1,SSC2) is used in the first SSS, the second SSS swaps the first SSC SSC1and the second SSC SSC2 with each other and thus uses a combination of(SSC2, SSC1).

Scrambling is performed by using two scrambling codes, corresponding tothe number of SSCs included in one SSS.

In the PSC 1 of FIG. 15, the first SSC SSC1 of the first SSS uses ascrambling code P1-al, the second SSC SSC2 of the first SSS uses ascrambling code P1-a2, the second SSC SSC2 of the second SSS uses ascrambling code P1-a1, and the first SSC SSC1 of the second SSS uses ascrambling code P1-a2.

In the PSC 2 of FIG. 16, the first SSC SSC1 of the first SSS uses ascrambling code P1-a2, the second SSC SSC2 of the first SSS uses ascrambling code P2-a2, the second SSC SSC2 of the second SSS uses ascrambling code P2-a1, and the first SSC SSC1 of the second SSS uses ascrambling code P2-a2.

In the PSC 3 of FIG. 17, the first SSC SSC1 of the first SSS uses ascrambling code P2-a2, the second SSC SSC2 of the first SSS uses ascrambling code P1-a1, the second SSC SSC2 of the second SSS uses ascrambling code P3-a1, and the first SSC SSC1 of the second SSS uses ascrambling code P3-a2.

From the viewpoint of physical subcarrier mapping, the two SSCs swaptheir locations for the first SSS and the second SSS, but do not swaplocations of the scrambling codes.

In this method, scrambling codes associated with three PSCs aredifferent from one another with respect to both the first SSC SSC1 andthe second SSC SSC2. This can reduce ambiguity and also bring aninterference randomization effect. For example, assume that an SSCcombination of a first SSS of a cell A is (P1-a1

SSC1_A, P1-a2

SSC2_A), an SSC combination of a second SSS of the cell A is (P1-a1

SSC2_A, P1-a2

SSC1_A), an SSC combination of a first SSS of a cell B is (P1-a2

SSC1_B, P2-a2

SSC2_B), an SSC combination of a second SSS of the cell B is (P1-a2

SSC2_B, P2-a2

SSC1_B), the cell A is a cell where a UE is currently located, and thecell B is a neighbor cell. Then, interference of the first SSS of thecell A is (P1-a2

SSC1_B, P2-a2

SSC2_B), and interference of the second SSS is (P1-a2

SSC2_B, P2-a2

SSC1_B). In practice, since a different code acts as interference to thefirst SSC SSC1 and the second SSC SSC2 with respect to the first SSS andthe second SSS, advantages of an interference averaging effect andmulti-frame averaging can be achieved without deterioration.Accordingly, detection performance on the SSSs can be improved.

In the example described above in which six or three scrambling codesare used for three PSCs, only SSC swapping have been described tofacilitate explanation. However, in addition thereto, differentialmodulation may be performed, and the SSC swapping may be performed incombination of the differential modulation. For example, the same mayalso apply to various cases, such as, in a case where the first SSS uses(SSC1, SSC2) and the second SSS uses (SSC2, SSC1), in a case where thefirst SSS uses (SSC1, SSC2) and the second SSS uses (SSC3, SSC4), in acase where the first SSS uses (SSC1, SSC2) and the second SSS uses(SSC1, SSC3), and a case where the first SSS uses (SSC1, SSC2) and thesecond SSS uses (SSC3, SSC2). When the first SSS uses (SSC1, SSC2) andthe second SSS uses (SSC1, SSC3), SSC1 collision occurs. Influenceresulted from the SSC collision can be reduced by swapping scramblingcodes. The same also applies in a case where SSC2 collision occurs whenthe first SSS uses (SSC1, SSC2) and the second SSS uses (SSC3, SSC2).

VI. Method for Configuring Scrambling Codes

Any code in association with a PSC can be used as a scrambling code.Technical features of the present invention are not limited thereto.

The scrambling code may be a PN code used in a SSC.

If the number of pieces of information transmitted on a SSS is 340, theSSC can be configured in the following manner. For example, if it isassumed that a PN code having a length of 31 is used for a first SSCSSC1 and a second SSC SSC2, available code indices are 0 to 30, that is,a total of 31 indices. If the first SSC SSC1 uses indices 0 to 13, thesecond SSC SSC2 uses indices 14 to 27, and the first SSC SSC1 and thesecond SSC SSC2 can be swapped, then the number of possible combinationsis 14×14×2=392. Therefore, a PN code having indices 28, 29 and 30 can beused as the scrambling code. For another example, it is possible toallow an index of the second SSC SSC2 to be always greater than an indexof the first SSC SSC1. If the first SSC SSC1 has indices 0 to 17, thesecond SSC SSC2 has indices 1 to 18, and the first SSC SSC1 and thesecond SSC SSC2 can be swapped, then the number of possible combinationsis 19C₂×2=342. Therefore, if six indices out of the remaining indices 19to 30 are selected, six scrambling codes can be acquired. If threeindices are selected, three scrambling codes can be acquired.

Now, assume that the number of pieces of information transmitted on theSSS is 680. If the index of the second SSC SSC2 is always greater thanthe index of the first SSC SSC1, the number of possible combinations is27C₂×2=702 when the first SSC SSC1 has indices 0 to 26, the second SSCSSC2 has indices 1 to 27, and swapping between the first SSC SSC1 andthe second SSC SSC2 is used. Accordingly, three scrambling codes can beacquired by selecting three indices among indices 28 to 30.

A scrambling code is selected from a currently used sequence set.Alternatively, a sequence is selected from the currently used sequenceset and thereafter the sequence is altered to be used. For example, whenan m-sequence is used, the m-sequence may be used as a scrambling codeby using a reverse operation, truncation, cyclic extension, cyclicshift, etc. That is, in the Equation 4, a sequence of (1) and a sequenceof (2) have a reverse relationship to each other. In this case, thesequence of (1) can be used as an SSC, and the sequence of (2) can beused as a scrambling code. When a pair of sequences having a reverserelationship is selected as a scrambling code, the SSC and thescrambling code can maintain an m-sequence relation. In addition,implementation is easy, and memory space can be saved.

FIG. 18 is a graph showing a cumulative distribution function (CDF) ofcross-correlation distribution for all possible collisions in two cells.

Referring to FIG. 18, the proposed method shows a similar characteristicof a random binary code. However, to use the random binary code as ascrambling code, a code generator or a memory is additionally required.On the contrary, the proposed method does not produce an additionaloverhead. This is because the proposed method requires onlyreconfiguration of a memory address.

Now, assume that an m-sequence of a polynomial x⁵+x²+1 of Equation 1 isused as an SSC. In order for a UE to detect the SSC, the sequence has tobe directly stored in a code generator or a memory capable of generatingthe sequence used in the SSC. The m-sequence generated by Equation 1 issubjected to cyclic shifting to acquire a total of 31 sequences. Insteadof generating each SSC detection code by the code generator, if onem-sequence is stored in a memory and only a memory address is assignedand used, then only one m-sequence having a length of 31 needs to bestored in the memory. If the sequence is used in a reverse order, onlyan order for indicating the memory address needs to be changed and used.

For example, assume that an m-sequence generated by Equation 1 isexpressed as (a)={1, 1, −1, 1, −1, −1, 1, 1, −1, −1, −1, −1, −1, 1, 1,1, −1, −1, 1, −1, −1, −1, 1, −1, 1, −1, 1, 1, 1, 1, −1}. An equivalentsequence can also apply to −(a). The remaining 30 sequences can begenerated by performing cyclic shifts 30 times on the sequence (a).Thus, only a memory for storing the sequence (a) is needed. To usesequences having a reverse relationship, it is sufficient to operate,only one time, the memory for storing the sequence (a) or the codegenerator for generating the sequence (a).

However, if the sequence is not reversed and other types of sequences(e.g., a random sequence, a computer search sequence, etc.) other thanthe m-sequence are used, a memory for storing six scrambling codesassociated with three PSCs is additionally required. That is, althoughit is sufficient to store one sequence having a length of 31 in thememory when reversely-related sequences are used, the memory for storingsix sequences having a length of 31 is additionally required whendifferent sequences are used.

In the selection of a scrambling code, an excellent feature can beachieved by generating an m-sequence after selecting a polynomialsatisfying Equations 2 and 3 above (or after reversing the order of thecoefficients of the polynomial in the order n-k). When the m-sequencegenerated by x⁵+x²+1 of Equation 1 is reversed, the sequence isconverted into one of m-sequences generated by x⁵+x³+1, which is calledas a pair relationship. For example, when a sequence, which is generatedby x⁵+x²+1 and is cyclic-shifted 0 times, is reversed, the resultingsequence is identical to a sequence which is generated by x⁵+x³+1 and iscyclic-shifted 26 times. Thus, when a pair of sequences having a reverserelationship is selected as a scrambling code, the UE can be easilyimplemented, and memory capacity can be saved.

VII. SSC1-Based Scrambling of SSC2

Now, an application of determining a scrambling sequence used in an SSC2according to a sequence index used in an SSC1 (i.e., application ofreverse-m) will be described.

In order to solve an additional ambiguity problem when a neighbor cellis searched for, there is a method for selecting and using a scramblingsequence one-to-one corresponding to a sequence index used in a firstSSC1, wherein a combination of two codes (e.g., (SSC1, SSC2)) is used ina SSS. In this case, for example, regarding the aforementioned 31-lengthm-sequence (31 sequence indices are possible) using the polynomialx⁵+x²+1, a sequence corresponding to an index of the m-sequence may bereversed to be used. For example, if the index of the SSC1 is 0, thesequence may be reversed to be used as a scrambling code for the SSC2.Alternatively, when SSC1-based SSC2 scrambling is used, all or someparts of the sequence used in the SSC1 may be reversed to be used as ascrambling code. In summary, a sequence used in the SSC1 can be reversedto be used as the scrambling code of the SSC2. This is not limited tothe number of scrambling codes, a one-to-one mapping relation, etc. Inaddition, polynomials in a reverse relationship can be selected.

Now, a case where the aforementioned description applies to [PSC-basedscrambling+SSC1-based scrambling] is disclosed.

Since a reverse-m is applied to SSC1-based scrambling in this case, forconvenience, a PSC-based sequence may use a 63-length m-sequence andpunctures the sequence if necessary or may use two different types ofm-sequences of a different polynomial. The scrambled SSC can beexpressed as follows.P

(SSC1,SSC2)=P

(si, sj),or (P

SSC1, P

SSC2)=(P

si, P

sj),or (P1

SSC1, P2

SSC2)=(P1

si, P2

sj)

Herein, P denotes a PSC-based scrambling code. Note that P does notchange whether scrambling is performed on the whole parts of an SSC orwhether scrambling is performed individually on each part of the SSC.

The SSC1-based scrambling is applied to a SSC2, as expressed by thefollowing expression.P

(SSC1, SCR1

SSC2)=P

(si, rev(si)

sj),or (P

SSC1, SCR1

P

SSC2)=(P

si, SCR1

P

sj),or (P1

SSC1, SCR1

P2

SSC2)=(P1

si, SCR1

P2

sj)

Herein, SCR1 denotes a SSC1-based scrambling code and rev(•) denotesrevere operation (or a reverse-m). Of course, as described above, theoperation is equivalent to the selection and the use of polynomials(herein, x⁵+x³+1) having a reverse relationship.

In the present example, si is directly reversed to be scrambled to sj.However, the present invention is not limited thereto, and thus areversely-related polynomial or a reversely-related sequence may also bedefined and used as a scrambling code.

When the SSC1-based scrambling is applied to the SSC2, a combinationform, such as, the aforementioned PSC-based scrambling+SSC1-basedscrambling, can be used.

The reverse-m of the present invention may be used, as described above,to the PSC-based scrambling scheme alone, the SSC1-based scramblingscheme alone, either one of the two scrambling schemes, or both of thetwo scrambling schemes.

VIII. Cell Search

A cell search is the procedure by which a UE acquires time and frequencysynchronization with a cell and detects the cell identity of the cell.In general, cell search is classified into initial cell search, which isperformed in an initial stage after a UE is powered on, and non-initialcell search which performs handover or neighbor cell measurement.

The cell search uses a PSS and a SSS. The PSS is used to acquire slotsynchronization (or frequency synchronization) and a unique identity.The SSS is used to acquire frame synchronization and a cell identitygroup. A cell identity for the cell is acquired by the unique identitywithin the cell identity group.

FIG. 19 is a flowchart showing a cell search procedure according to anembodiment of the present invention.

Referring to FIG. 19, a UE searches for a PSS (step S310). A UEidentifies a PSC by the PSS transmitted from a base station. Slotsynchronization are acquired by using the PSS. Frequency synchronizationcan also be acquired by using the PSS. A PSC in the PSS is associatedwith an unique identity. When there are 3 unique identities, each of 3PSC is one-to-one mapped to the each of the unique identities.

Next, the UE searches for a SSS (step S320). The UE identifies two SSCsby the SSS transmitted from the base station. Frame synchronization isacquired by using the SSS. The SSS is mapped to a cell identity group.By using the SSS and the PSS, cell identity is acquired. For example, itis assumed that there are 504 unique cell identities, the cellidentities are grouped into 168 unique cell identity groups and eachgroup contains three unique identities. 3 PSSs are respectively mappedto the three unique identities and 168 SSSs are respectively mapped to168 cell identity groups. A cell identity I_(cell) can thus be uniquelydefined by a number I_(gr) in the range of 0 to 167, representing thecell-identity group, and a number I_(u) in the range of 0 to 2,representing the unique identity within the cell-identity group as shownI_(cell)=3 I_(gr)+I_(u).

The SSS includes two SSCs. Each SSC is scrambled by using differentscrambling codes. The scrambling code is associated with the PSCincluded in the PSS. Therefore, cell search can be performed much fasterby reducing interference of a neighbor cell and by improving SSSdetection performance.

Detection performance of a SSS can be improved by scrambling two SSCs inthe SSS using different scrambling codes. Cell search can be performedmore reliably and can be prevented from being delayed. In addition, withan increase in the number of available sequences, an amount ofinformation carried by the synchronization signals and capacity of auser equipment can be increased.

Although synchronization signals have been described above, technicalfeatures of the present invention may also apply to other signal whichdelivers information in order to improve channel detection performance.For example, this can apply to an uplink/downlink reference signal, anACK/NACK signal, a random access preamble, etc.

All functions described above may be performed by a processor such as amicroprocessor, a controller, a microcontroller, and an applicationspecific integrated circuit (ASIC) according to software or program codefor performing the functions. The program code may be designed,developed, and implemented on the basis of the descriptions of thepresent invention, and this is well known to those skilled in the art.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method of performing cell search in a wirelesscommunication system, performed by a user equipment, the methodcomprising: searching a primary synchronization signal (PSS) comprisinga primary synchronization code (PSC); searching a first secondarysynchronization signal (SSS) comprising a first secondarysynchronization code (SSC) and a second SSC, searching a second SSScomprising the first SSC and the second SSC, wherein the first SSC ofthe first SSS is scrambled by using a first scrambling code, the secondSSC of the first SSS is scrambled by using a second scrambling code, thefirst SSC of the second SSS is scrambled by using the second scramblingcode, and the second SSC of the second SSS is scrambled by using thefirst scrambling code, wherein the first and second SSCs are defined bym-sequences generated based on a first polynomial and the first andsecond scrambling codes are defined by m-sequences generated based on asecond polynomial, wherein the first polynomial and the secondpolynomial satisfy reverse relationship.
 2. The method of claim 1,wherein the second SSC of the second SSS is further scrambled by asequence.
 3. The method of claim 2, wherein the sequence is generatedfrom binary phase shift keying (BPSK) modulation.
 4. The method of claim1, wherein the first scrambling code and the second scrambling codedepend on the PSS.
 5. The method of claim 1, wherein the first andsecond SSCs are generated by using different cyclic shifts.
 6. Themethod of claim 1, wherein the first and second scrambling codes aregenerated by using different cyclic shifts.
 7. A user equipmentconfigured for performing cell search in a wireless communicationsystem, comprising a processor configured to: search a primarysynchronization signal (PSS) comprising a primary synchronization code(PSC); search a first secondary synchronization signal (SSS) comprisinga first secondary synchronization code (SSC) and a second SSC; andsearch a second SSS comprising the first SSC and the second SSC, whereinthe first SSC of the first SSS is scrambled by using a first scramblingcode, the second SSC of the first SSS is scrambled by using a secondscrambling code, the first SSC of the second SSS is scrambled by usingthe second scrambling code, and the second SSC of the second SSS isscrambled by using the first scrambling code, wherein the first andsecond SSCs are defined by m-sequences generated based on a firstpolynomial and the first and second scrambling codes are defined bym-sequences generated based on a second polynomial, wherein the firstpolynomial and the second polynomial satisfy reverse relationship. 8.The user equipment of claim 7, wherein the second SSC of the second SSSis further scrambled by a sequence.
 9. The user equipment of claim 8,wherein the sequence is generated from binary phase shift keying (BPSK)modulation.
 10. The user equipment of claim 7, wherein the firstscrambling code and the second scrambling code depend on the PSS.
 11. Abase station comprising a processor configured to: transmit a primarysynchronization signal (PSS) comprising a primary synchronization code(PSC); transmit a first secondary synchronization signal (SSS)comprising a first secondary synchronization code (SSC) and a secondSSC; and transmit a second SSS comprising the first SSC and the secondSSC, wherein the first SSC of the first SSS is scrambled by using afirst scrambling code, the second SSC of the first SSS is scrambled byusing a second scrambling code, the first SSC of the second SSS isscrambled by using the second scrambling code, and the second SSC of thesecond SSS is scrambled by using the first scrambling code, wherein thefirst and second SSCs are defined by m-sequences generated based on afirst polynomial and the first and second scrambling codes are definedby m-sequences generated based on a second polynomial, wherein the firstpolynomial and the second polynomial satisfy reverse relationship. 12.The base station of claim 11, wherein the second SSC of the second SSSis further scrambled by a sequence.
 13. The base station of claim 12,wherein the sequence is generated from binary phase shift keying (BPSK)modulation.
 14. The base station of claim 11, wherein the firstscrambling code and the second scrambling code depend on the PSS.